A comparison of reproductive cycles and reproductive output in four southern African mussel species

The timing and intensity of spawning events in 4 southern African mussel species were deduced from fluctuations in the dry flesh weight of standard-sized individuals, as calculated from monthly length-weight regressions. The sex ratios in 3 of the 4 species (Aulacomya ater, Choromytilus meridionalis and Perna perna) were biased in favour of males, but this was not the case for Mytilus gallopiovincialis. The calorific value of females was higher than that of males in all species. A. ater spawned 3 times a year, although these spawning events differed considerably in timing and intensity between years. C. meridionalis and M. galloprovincialis generally showed 2 protracted spawning seasons, one each during summer and winter. P. perna from Natal spawned over an extended period from May/June to December, while those from the Cape and Transkei had a well-marked winter spawning, accompanied by either 1 extended, or 2 discrete, spring or summer events. Gamete build-up and release resulted in marked variations in flesh yield. This was greatest in M. galloprovincialis, where dry flesh weight can vary 3-fold between ripe and spawned conditions at a length of 65 mm. Equivalent values for A. ater, C. meridionalis and P. perna were 2.3-, 2.8- and 2.2-fold respectively. Total individual reproductive output was greatest in A. ater, because of its 3 annual spawnings, followed by M. galloprovincialis, C. meridionalis and P. perna. When extrapolated to a population level, beds of mussels were estimated to release 3.4 to 10.4 kg wet mass gametes m−2 yr−1. This represents a large energy subsidy to adjacent benthic and pelagic communities.